1. Field of the Invention
The present invention relates to a direct-injection internal combustion engine and method for controlling the engine wherein stratified combustion is caused.
2. Description of the Related Art
For the purpose of improving a fuel consumption rate, there has been developed a direct-injection internal combustion engine wherein stratified combustion is caused. In the process of stratified combustion, a rich mixture and lean mixture are formed in a combustion chamber in a layered manner, and the rich mixture is first ignited so that the flames generated thereby burn the lean mixture. Thus, the mixture which is lean as a whole burns with the possibility of incomplete combustion or misfiring being removed. When stratified combustion is caused, fuel injection is carried out in a compression stroke. On the other hand, when homogeneous combustion for burning a stoichiometric mixture is caused, fuel injection is carried out in an intake stroke.
In this manner, the direct-injection internal combustion engine has mutually different operation modes, namely, stratified combustion and homogeneous combustion. Depending on which of these operation modes is selected, a load detecting parameter of an operating condition is detected either from an accelerator operation amount or from an amount relating to intake air, such as a flow rate, a negative pressure, a throttle opening degree or the like. In such circumstances, the control regarding a fuel injection amount and ignition timing and the control regarding an amount of intake air and EGR must be designed to differ from each other in responding performance (e.g. see Japanese Patent Application Laid-Open No. HEI 5-99020).
When one of the operation modes is selected, a fuel injection amount and a control amount for ignition timing are directly calculated using a load detecting parameter (without making any correction). When the other operation mode is selected, the calculation is made with a necessary correction made. Thus, the operation of the engine is optimized in each of the operation modes. However, when making a transition to the operation mode where a correction is made, an appropriate correction amount cannot be used immediately. For this reason, it is difficult to switch operation modes from one to the other smoothly.